Descriptions

The measurement of phytoplankton carbon (C[subscript]phyto) in the field has been a long-sought but elusive goal in
oceanography. Proxy measurements of C[subscript]phyto have been employed in the past, but are subject to many
confounding influences that undermine their accuracy. Here we report the first directly measured C[subscript]phyto
values from the open ocean. The C[subscript]phyto samples were collected from a diversity of environments, ranging
from Pacific and Atlantic oligotrophic gyres to equatorial upwelling systems to temperate spring conditions.
When compared to earlier proxies, direct measurements of C[subscript]phyto exhibit the strongest relationship with
particulate backscattering coefficients (b[subscript]bp) (R²=0.69). Chlorophyll concentration and total particulate
organic carbon (POC) concentration accounted for ~ 20% less variability in C[subscript]phyto than b[subscript]bp. Ratios of C[subscript]phyto
to Chl a span an order of magnitude moving across and within distinct ecosystems. Similarly, C[subscript]phyto:POC
ratios were variable with the lowest values coming from productive temperate waters and the highest from
oligotrophic gyres. A strong relationship between C[subscript]phyto and b[subscript]bp is particularly significant because b[subscript]bp is a
property retrievable from satellite ocean color measurements. Our results, therefore, are highly encouraging
for the global monitoring of phytoplankton biomass from space. The continued application of our C[subscript]phyto
measurement approach will enable validation of satellite retrievals and contribute to an improved
understanding of environmental controls on phytoplankton biomass and physiology.